The invention relates to a piece of equipment to be installed on the sea bed for subsea oil well drilling. More specifically, the object of invention is to provide a single structure to be installed on the sea bed, intended to serve as a base for subsea oil well drilling work.
The discovery of oil fields lying within continental shelves has led to the development of new techniques in the drilling of oil wells. Throughout the years new equipment and methods of work have been developed by the specialists for the purpose of facilitating the difficult task of drilling for subsea oil.
Starting from simple adaptations of equipment used for onshore work, subsea drilling techniques have developed to the point of having to call into play highly complex equipment, very different from that employed onshore, and requiring the efforts of specialists endowed with a high degree of technical knowledge.
Among the various kinds of equipment developed for enabling subsea oil drilling to take place, those known as drilling guide bases are a typical example of the progress that has been made in subsea drilling techniques. A drilling guide base is fastened to the first casing of a well driven in the sea bed and serves several purposes including the guiding of other nearby well casings to ensure a vertical well. The consequences can be serious if a drilling guide base fails to ensure that the various casings are properly vertical.
The first guide bases developed to meet the requirements of oil subsea drilling work comprised largely two structures.
The first was referred to by the specialists as a temporary guide base and was laid on the sea bed coupled to the well pipe casing. Though it is provided with devices for offseting tilting, its natural tendency is to match unevenness in the sea bed, which is of course not desirable because in doing so it will make vertical well drilling difficult.
As a way of overcoming this drawback a second structure, referred to by the specialists as a permanent guide base, was laid over the temporary one. This second guide base had devices intended to offset any tilting of the temporary guide base.
Brazilian application PI 8700104-7 shows a typical example of a guide base made up of temporary and permanent bases, where the permanent guide base rests on the temporary guide base by means of a semi-spherical surface correcting any tilting that may have taken place when the temporary guide base was laid.
Brazilian application PI 8900855-3 introduced an inovation by proposing that a single guide base be employed to guide subsea drilling operations, and enabling vertical wells to be drilled regardless of the unevenness of the sea bed.
This single guide base comprises a cylindrical structure which has in its upper, part a funnel to guide the descent of the pieces of equipment into the well. To give strength to the assembly and to enable it to be put in a vertical position, a driven pile is jetted into the sea bottom, all of this operation being monitored by television in a remote operated vehicle; the slope of the assembly being controled with the aid of a level indicator.
The next step is to install the low-pressure housing with its casing. This low-pressure housing is placed upon another housing previously bolted to,the base. Then the casing is cemented.
Next the well is drilled, after which the head pressure housing and its respective casing set and finally cementing is carried out, all of which provides a firm base to support the other casings to be installed in the well.
This single guide base has brought about great progress in well drilling techniques and has led to considerable savings because it has done away with certain parts and expensive operations. However, there are still some shortcomings, of which the principal one concerns the large size of the arrangement which makes hauling and installing very difficult.